The present invention relates to a process for electrolytic recovery of zinc from zinc sulfate solutions according to the electrowinning principle, using an aluminum cathode.
It is previously known to recover zinc electrolytically according to the electrowinning principle by using a silver-bearing lead anode and as the electrolyte a zinc sulfate solution which contains zinc 50-65 g/l and sulfuric acid 100-180 g/l. The cathodes used in this case are aluminum sheets on which zinc is deposited electrolytically. The zinc is allowed to accumulate on the aluminun sheets for 24 h, operating with a current density of 450-600 amp/m.sup.2, which has been found in practice to be good. Thereafter, the cathodes are lifted out and the zinc is detached from them. Finally the zinc plates are fed, together with slagging ammonium chloride, into the casting furnace for the casting of zinc bars.
When the objective is to deposit pure zinc, keeping the power supply as high as possible, the traditional method is to use as pure electrolytic solutions as possible. It has been a general belief that Ge, Sb, As, Se, Fe, Co and Ni have an especially adverse effect on zinc electrolysis. A careful removal of all impurities from the solutions is, however, expensive and makes the process uneconomical.
When zinc is precipitated from an impure solution, zinc first deposits as an even layer on the cathode surface. After some time the surface begins to grow unevenly. So-called dendrites (FIG. 1) are formed on the surface. Impurities, which usually have a lower hydrogen overvoltage than zinc, deposit around the dendrites. The spot-like difference in voltage between the impurity deposit and the zinc deposit results in that, when impurities deposit, zinc begins to pass back into the solution, and at the same time hydrogen is generated. The total current afficiency .eta..sub.tot is the sum of the zinc current efficiency .eta..sub.Zn and the hydrogen current efficiency .eta..sub.H, i.e. .eta..sub.tot =.eta..sub.Zn +.eta..sub.H.
Since hydrogen is produced in the "miniature electrolysis" occurring at the impurity sports around the dendrites, the current efficiency of zinc is lowered. The effect of these reactions becomes so important that it is futile to continue the electrolysis, and the cathodes are lifted out of the solution.
In attempts at preventing impurities such as dendrites from depositing on the cathode surface, various oganic compounds are generally added to the solution, but also "neutral" inorganic compounds such as sodium silicate, Na.sub.2 SiO.sub.3. The effect of the additives, preventing the growth of dendrites, is explained to be due to the adsorption of the additive to the cathode surface, whereby the growth of Zn crystals is prevented and new nucleation spots are produced. Thereby the crystal structure of the zinc becomes finer and the surface more even. Another aim in using additives is the formation of a foam which prevents evaporation on the surface of the electrolytic tank. However, practice has shown that additives also decrease the current efficiency, and especially if longer growth periods are the aim, maintaining a high current supply is very difficult.
In known processes, efforts are made to maintain, as low as possible, the impurity content in the solution entering the Zn electrolysis, for example, Co and Ni within the range 0.1-0.2 mg/l. Electrolytic Zinc Co of Australasia uses an electrolytic solution which contains 10 mg/l Co, but the cobalt is combined in an organic complex (.alpha.-nitroso-.beta.-naphthol), and so cobalt is not actually in the solution and consequently the crystal structure and the surface quality are similar to those in a normal system.
The object of the present invention is, therefore, to provide a process for electrolytic recovery of zinc from zinc sulfate solutions, with an improved current supply.